So I'm sitting there looking up at the random stacks of computer hardware I have in my loft. A couple of NeXTstations, a few Apple IIs, half a dozen Macs from the 90s....and as I'm looking I come upon this old IBM monitor that I saved from destruction several years ago.

So I got to thinking that I would turn this into an oscilloscope.

Here's the monitor, an IBM PCJr color monitor from 1985.

To start, off, here is basically how a CRT display basically works:

A gun at the back of the screen shoots out a beam of electrons towards a fluorescent screen, and two electromagnetic deflector coils move the beam up and down 60 times a second, and side to side 31500 times a second, to form an image on the screen.

So here is the back all opened up on our IBM monitor.

Deflector Coil Assembly

Basically, there are four wires leading up to two deflector coils in the neck of the CRT. In this case, red and blue control the horizontal sync while yellow and brown control the vertical sync. You can find out which is which by disconnecting them and reconnecting them, and seeing what happens to the picture. If you get a vertical line, you probably just cut one of the horizontal signal wires, and vice-versa.

After some trial and error, I figured that by keeping the vertical sync connected, and replacing the horizontal sync with an audio signal from an amplifier, you get a nice little oscilloscope:

Vertical Configuration (1 height = 60Hz)

I tried it reverse too; replacing the vertical sync with the audio input and keeping the horizontal sync connected to the monitor supply, but this didn't work as well. I ended up with a horizontal line that would just move up and down with the amplitude of the music. It didn't really show any waves.

 
Horizontal Configuration (1 width = around 30KHz)

This is because the original vertical sync signal was 60Hz, while the original horizontal sync signal was something like 31500Hz (60Hz times 525 lines). So the vertical configuration would show a 60Hz wave from peak to valley (picture below), and other waves in the audible spectrum, with 120Hz being half of that, 240Hz being half of that and so on.

60Hz sine wave on the vertical oscilloscope

I also looked up the pinout of the PCJr proprietary monitor connector so I could change the colors. You short the intensity pin to the color that you want. The below example gives me red.

PCjr display connector

At some point, I might make a circuit that changes the colors with the amplitude of the music. Which would be pretty neat.

Here are some shots of the oscilloscope that I took in a dark room:

60Hz sine wave	120Hz sine wave	240Hz sine wave

And there you have it. It's not a replacement for an actual calibrated scope, but it certainly looks cool. Here's a video of the oscilloscope in action:

 And below is a later video I made of it playing Bohemian Rhapsody by Queen